Fix pixel coordinate to tile coordinate conversion

This addresses issue geo-data#9. Although this does not appear to be an issue on Linux
for some reason, it affected a Windows build and the logic appears sound e.g `50
/ 100` correctly resolves to tile `0` whereas `ceil(50.0 / 100)` resolves to
tile `1`.

src/Grid.hpp

@@ -117,8 +117,8 @@ class ctb::Grid {
   /// Get the tile covering a pixel location
   inline TilePoint
   pixelsToTile(const PixelPoint &pixel) const {
-    i_tile tx = (i_tile) ceil(pixel.x / mTileSize),
-      ty = (i_tile) ceil(pixel.y / mTileSize);
+    i_tile tx = (i_tile) (pixel.x / mTileSize),
+      ty = (i_tile) (pixel.y / mTileSize);
 
     return TilePoint(tx, ty);
   }